The present invention relates to a ferroelectric memory and a piezoelectric device formed by using a ferroelectric capacitor. The present invention also relates to a ferroelectric film which can be used in common for 1T1C and 2T2C type ferroelectric memory devices which include a ferroelectric capacitor and a select cell transistor, a simple matrix type ferroelectric memory device in which a memory cell is formed by using only a ferroelectric capacitor without a cell transistor, and a 1T type ferroelectric memory device in which a ferroelectric film is used as a gate oxide film, a method of manufacturing the ferroelectric film, a ferroelectric memory, and a piezoelectric device.
In recent years, research and development of a film such as PZT or SBT, a ferroelectric capacitor using the film, and a ferroelectric memory device have been extensively conducted. The structure of a ferroelectric memory device is roughly divided into 1T, 1T1C, 2T2C, and simple matrix. Since the 1T type ferroelectric memory device has a structure in which retention (data retention) is as short as one month due to occurrence of an internal electric field in the capacitor, it is impossible to secure a 10-year guarantee generally required for semiconductors. The 1T1C and 2T2C type ferroelectric memory devices have almost the same configuration as that of a DRAM, and include a select transistor. Therefore, a DRAM manufacturing technology can be utilized and a write speed approximately equal to the write speed of an SRAM can be realized. Therefore, small capacity products with a capacity of 256 kbits or less have been produced on a commercial basis.
As a ferroelectric material used for a ferroelectric memory, a perovskite material such as Pb(Zr,Ti)O3 (PZT) and a Bi-layered-structured ferroelectric such as Bi4Ti3O12 (BIT) have been used. However, since these materials have a leakage current density of about 10−4 to 10−6 A/cm2, an extremely large current leakage occurs.
PZT is used as a typical ferroelectric material. A material having a composition in or near the mixed region of the rhombohedral crystal and the tetragonal crystal, in which the Zr/Ti ratio is 52/48 or 40/60, is used as PZT. PZT is used after doping with an element such as La, Sr, or Ca. The above region is used to secure reliability, which is the most important requirement for the memory device. Although the hysteresis shape is excellent in the Ti-rich tetragonal region, a Schottky defect occurs due to the ionic crystal structure, whereby a failure in leakage current characteristics or imprint characteristics (degree of change in hysteresis shape) occurs. This makes it difficult to secure reliability.
In recent years, it is known that the crystallization temperature is reduced by adding Si and Ge to the constituent elements of the ferroelectric crystal in order to solve the above-described problems. However, it is not known which site Si and Ge replace in the crystal. This is an important subject in the functional design of the ferroelectric material in the case of introducing an element other than Si and Ge.
The simple matrix type ferroelectric memory device has a cell size smaller than that of the 1T1C and 2T2C type ferroelectric memory devices, and allows the capacitors to be multilayered. Therefore, an increase in the degree of integration and a reduction of cost are expected by using the simple matrix type ferroelectric memory device. A conventional simple matrix type ferroelectric memory device is disclosed in Japanese Patent Application Laid-open No. 9-116107, for example. Japanese Patent Application Laid-open No. 9-116107 discloses a drive method in which a voltage of one-third the write voltage is applied to unselected memory cells when writing data into the memory cell.
However, this technology does not describe the hysteresis loop of the ferroelectric capacitor necessary for the operation in detail. The present inventors have developed a ferroelectric memory device and found that a hysteresis loop with excellent squareness is indispensable for obtaining a simple matrix type ferroelectric memory device which can be operated in practice. As a ferroelectric material which can deal with such a requirement, Ti-rich tetragonal PZT may be employed. However, the most important subject is to secure reliability in the same manner as the above-described 1T1C and 2T2C type ferroelectric memories.